Silicone elastomers can be made from many different polyorganosiloxanes, cure mechanisms and fillers. These silicone elastomers have been made from stiff gum polymers as well as water thin fluids. The curing mechanisms have ranged from organic peroxides to moisture sensitive means to radiation. A variety of fillers have also been used, such as reinforcing silica fillers and extending fillers.
Presently, silicone elastomeric compositions are usually prepared from a vinyl-containing polydiorganosiloxane, an organohydrogensilicone crosslinker, a platinum catalyst, and, optionally, fillers. The compositions of this type are desirable for many reasons. For instance, they cure without by-products; thus, there is no need for stripping or devolitilization. They can cure at room temperature or at elevated temperatures. They can be stabilized for storage at room temperature by utilization of a platinum catalyst inhibitor. And, they can be made from high and low viscosity polymers. These compositions utilize components that are low enough in viscosity that they are easily pumpable or extrudable as well as have a fast cure time. These compositions also provide cured silicone elastomers which are characterized by high strength and high durometer.
These platinum catalyzed silicone elastomers are generally two package systems. One package generally contains the base polymer (which itself contains vinyl unsaturation) and the platinum catalyst. The other package generally contains the base polymer and the organohydrogensilicone. When the two packages are mixed together, the platinum catalyzes the addition of the organohydrogensilicone to the vinyl containing base polymer. Heat sensitive inhibitors are generally used to prevent the catalyst from polymerizing the composition at room temperature. Elevated temperatures are then employed to neutralize the inhibitor and allow the reaction to proceed.
The problems with these silicone elastomers, however, is their relatively high compression set which has prevented such elastomers from being used in sealing applications where high temperature resiliency is an important factor. An example of this would include automotive gasketing and similar applications. The compression set of LIM silicone elastomers is generally well above that useful for applications such as gasketing. This problem with compression set stems from the LIM process itself. In order to decrease cycle times, molders will remove the elastomer from the mold prior to final cure. Therefore, in order to improve compression set, another step, post bake, is required where the elastomer is further cross-linked by heating.
In addition, automotive gasket applications would also require the use of a gasket material that also has fuel and oil resistance as well as low compression set without any sacrifice of durometer or strength. Silicone elastomers, however, are not known for their fuel/oil resistance.
Further, although platinum catalyzed addition reactions are possible at room temperature, inhibitors are often employed to stabilize the compositions until needed. These inhibitors are heat sensitive and are designed to be ineffective at elevated temperatures. Molders of these products, however, are requiring lower and lower reaction temperatures in their processes, requiring development of reaction mechanisms which are still stable at room temperature yet will react at only moderately elevated temperatures. Finally, molders also would like to see a one-part composition that requires no mixing of the catalyst and organohydrogensilicone crosslinker.
Accordingly, it is highly desirable to have a silicone rubber composition which cures quickly, if desired, at elevated temperatures and would require no post baking as is traditional with heat vulcanizable silicone rubber compositions.
It is also desirable to prepare a silicone rubber composition which can be injection molded. Most silicone rubber compositions and particularly heat vulcanizable silicone rubber compositions, because of their high viscosity, are very difficult to injection mold and require excessively high pressure such as, 40,000 psi and even higher. In addition, such heat vulcanizable silicone rubber compositions are difficult to injection mold so as to form intricate parts. They require special molds that are to be used in the injection molding process as well as long times to cure at elevated temperatures.
Accordingly, it is one object of the present invention to provide a liquid injection moldable (LIM), elastomeric silicone rubber composition which in the cured state has a combination of low compression set and resistance to fuel and oil as well as high tensile strength, high elongation, and high tear.
It is further an object of this invention to provide an elastomeric silicone rubber composition which requires no post bake step.
It is further an object of this invention to provide a one part LIM elastomeric composition.
It is further an object of this invention to provide an elastomeric silicone rubber composition which cures at a temperature lower than is normally used to cure such compositions.
These and other objects of the present invention are accomplished by means set forth hereinbelow.